Some time in the middle of October my instrument rating became a lot less useful, and it will stay that way until some time in the middle of next April. For half the year, the rating is almost a guarantee that I can fly. Sure, sometimes there’s a solid squall line that I cannot get around, and sometimes the weather is below my personal IFR minima, but last year, out of dozens of planned flights during the late-spring/summer/early-fall season, I had to postpone exactly one due to weather (a night flight with numerous thunderstorms enroute) — that’s better than I’ve done in the past travelling for business on the airlines. Come October, though, I might almost as well be VFR only.
Once the freezing levels come down low enough, clouds can mean ice, and most single-engine planes (and many twins) are not equipped to deal with that. It doesn’t have to be winter weather for icing to be a problem: with the standard temperature lapse rate (i.e. no inversion), a nice warm fall/spring temperature of 8 degC on the ground means that I’ll be hitting icing conditions at about 4,000 feet, which gives me very little room for IFR flight (the lowest IFR obstacle-clearance altitudes are typically around 3,000 feet, and I like a 2,000 foot safety margin from the icing). If the clouds are high enough, I can go VFR underneath them; if the ceiling is low, I might just have to wait.
Actually, the story is more complicated than that. I will admit that last winter — my first with an instrument rating — I flew a lot in cloud at below-freezing temperatures. The only ice I ever saw was a tiny piece of clear ice starting to form on my outside air temperature probe, in drizzle around -1 degC. I immediately descended 1,000 feet, the ice melted off, and I finished my flight uneventfully aside from a slightly fast, no-flaps landing. What if it hadn’t melted? Well, I could have descended further below MEA to MOCA (the minimum safe IFR altitude); if that didn’t work, I had already listened to the weather from the nearest big airport, which was reporting surface temperatures of around 6-8 degC (memory is getting fuzzy), and I had the navaids for a nearby approach tuned in, so I could have shot the approach until I broke out from the clouds and then decided whether to land or continue VFR underneath. In other words, I had more than one very good out. Would I have tried the same thing with freezing temperatures down to near the ground or over hilly or mountainous terrain? Not a chance. There’s always got to be a way out, preferably several.
Too cold for icing
More typically, though, I can fly in IMC in the winter without worrying, because as the winter goes on it gets too cold for airframe icing. According to this bulletin to FAA controllers, at -16 degC, 95% of droplets in a cloud will have frozen into ice crystals (which won’t stick to the airframe); at -25 degC, 99.9% of the droplets will have crystalized. I live in Ottawa, where people refer to -16 degC on the ground in February as a mild day; I’ve even flown on -16 degC days without preheating (I promise never to do it again). And, when there’s no inversion, -16 degC on the ground might mean -30 degC at altitude, where there are definitely nothing but ice crystals in the clouds. So come January and February, whenever Ottawa is under a cold arctic airmass, I can happily go and fly in the clouds without worrying about icing at all. However, when that warm wet gulf air mass pushes up from the U.S. and the surface temperature rises to, say, -5 degC or even 0 degC, I’m back to VFR only. Once the surface temperature gets above 5 or 6 degC, I can fly again, because I know that I’ve got warmer air underneath to get rid of any ice I might pick up. That means that the worst time for icing is not winter but late October, November, March, and early April, where the surface temperature is just a bit above freezing.
I think it’s a big shame that training courses for instrument ratings do not spend much more time on icing. Simply saying to an IFR pilot “Don’t fly in clouds below freezing” is roughly equivalent to saying to a VFR pilot “Don’t fly when there are clouds in the sky” or saying to a teenager “Don’t have sex” — they’re probably going to do it anyway, so you might as well teach them how to do it safely. Sometimes flying in icing conditions is OK and sometimes it’s not, and we really need more resources to help new instrument-rated pilots make more informed decisions and develop strategies for flying safely during the icing months.